Apodized UV-induced Gratings in Planar Waveguides for Compact Add-Drop Filters

Abstract

Add-drop filters are key components for WDM communication systems. Filters with flat passbands, which can separate channels spaced by 100 GHz, are needed to allow many devices to be concatenated without decreasing the available channel bandwidth significantly. A Mach- Zehnder Interferometer (MZI) with UV-induced gratings realized using planar waveguides [1] is an attractive architecture for creating an add-drop filter (see Fig. 1a). To increase the number of devices which can be fabricated on a common substrate, it is desirable to make the overall device as short as possible; however, requirements on the allowable channel crosstalk (typically -20 dB for adjacent channels and -30 dB between the dropped and added channels) places a lower limit on grating length. Previous work has demonstrated Gaussian profile gratings [2] which have sidelobes on the short wavelength side of the Bragg wavelength. A double filtering architecture was reported with a transmitted isolation of -40 dB over the drop channel width and a 3dB width of 1 nm [3]. Such an architecture increases the number of gratings needed for each channel and requires that the two grating passbands be identical or passband narrowing occurs.